bug#33255: 27.0.50; expand-file-name: default directory expanded twice if relative

[immerrr again]

I have recently tried to play with a clean .emacs.d directory by setting
HOME=clean/home/dir, but faced an error coming from deep inside
package-install. The error boils down to the fact that
`expand-file-name' for some reason doubles the `default-directory'
prefix when it is relative:


$ emacs -Q -batch --eval '(let ((default-directory "DEFAULT-DIR/"))
(print (expand-file-name "EXPANDED-DIR/")))'

"DEFAULT-DIR/DEFAULT-DIR/EXPANDED-DIR/"


Interestingly, if you run `expand-file-name' twice you end up with four
prefixes:


$ emacs -Q -batch --eval '(let ((default-directory "DEFAULT-DIR/"))
(print (expand-file-name (expand-file-name "EXPANDED-DIR/"))))'

"DEFAULT-DIR/DEFAULT-DIR/DEFAULT-DIR/DEFAULT-DIR/EXPANDED-DIR/"


I couldn't find any requirement for DEFAULT-DIRECTORY parameter or
`default-directory' variable to be absolute, so I would assume this is a
bug in expand-file-name.


In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2018-07-09 built on mmrcomp
Repository revision: 3307353e13a9226d477c9b1a39baae76584b90b9
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Ubuntu 16.04.5 LTS

Recent messages:
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Configured using:
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